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Bulk nanostructured (grain sizes in the range of 50-200nm) and ultrafine structured (grain sizes in the range of 100-500nm) -TiAl based alloys with compositions Ti-47Al (in at%) and Ti–45Al–2Cr–2Nb–1B–0.5Ta (in at%), respectively, have been produced using a combination of high energy mechanical milling of mixtures of elemental powders and hot isostatic pressing at 800 and 1000oC respectively, and the microstructures of the samples have been characterised. At room temperature, the HIPed samples fractured prematurely at tensile stresses in the range of 200-300MPa and showed no ductility, very likely due to the relative high oxygen content (0.6wt%) in the samples and very low tolerance of TiAl based alloys on dissolved oxygen. At 800oC, the HIPed samples showed a yield strength in the range of 55-70MPa, a tensile strength in the range of 60-80MPa, a large amount of elongation to fracturing around 100% and clear strain softening. Examination of the fractured tensile test specimens at room temperature and 800oC showed that the level of the consolidation was fairly high, but the HIPed samples do contain a small fraction of interparticle boundaries with weak atomic bonding. The fracture of the HIPed samples in tensile testing at room temperature and 800oC, respectively, is predominately intergranular, and the large amount of plastic deformation prior to fracture at 800oC is achieved mainly through grain boundary sliding in conjunction with dislocation gliding, in agreement with the deformation mechanisms of nanostructured and ultrafine structured alloys generally agreed by researchers.

Abstract: Cast Mg-4.27Y-2.94Nd-0.51Zr (wt.%) alloy was subjected to submerged friction stir processing (SFSP) with at a rotation rate of 600 rpm and a traveling speed of 60 mm min-1. Superplastic behavior of specimens with an average grain size of ~1.3 μm were investigated in the temperature ranges of 683-758 K and the strain rate ranges from 1×10-1 to 4×10-4 s-1. Microstructure characteristics were investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that the maximum elongation of 967% was obtained at 733 K and 3×10-3 s-1, the optimal HSRS of 900% achieved at 758 K and 2×10-2 s-1. Grains and second phase particles grew coarser with the increasing temperature or decreasing strain rate. Remarkable grain growth is the main reason that elongations are all significantly decreased when the strain rate decrease from 3×10-3 s-1 to 4×10-4 s-1. Grain boundary sliding is the main mechanism during superplastic deformation.

Abstract: Three Zn-Al alloys, namely Zn-22Al, Zn-5Al and Zn-0.3Al, were subjected to equal-channel angular pressing (ECAP), and the effect of ECAP on their microstructure and room temperature (RT) superplastic behavior were investigated in detail referring to previous studies reported by the authors of the current study. ECAP remarkably refined the microstructures of three alloys as compared to their pre-processed conditions. While the lowest grain size was achieved in Zn-22Al alloy as 200 nm, the grain sizes of Zn-5Al and Zn-0.3Al alloys were ~540 nm and 2 µm, respectively, after ECAP. After the formation of fine/ultrafine-grained (F/UFG) microstructures, all Zn-Al alloys exhibited superplastic behavior at RT and high strain rates. The maximum superplastic elongations were 400%, 520% and 1000% for Zn-22Al, Zn-5Al and Zn-0.3Al alloys, respectively. It is interesting to point out that the highest RT superplastic elongation was obtained in Zn-0.3Al alloy with the largest grain size, while Zn-22Al alloy having the lowest grain size showed the minimum superplastic elongation. This paradox was attributed to the different phase compositions of these alloys. The formation of Al-rich α/α phase boundaries, where grain boundary sliding is minimum comparing to Zn-rich η/η and η/α phase boundaries of Zn-Al alloys, is the lowest level in Zn-0.3Al alloy among all the alloys. Therefore, it can be concluded that if it is desired to achieve high superplastic elongation in Zn-Al alloys at RT, keeping Al content at a possibly minimum level seems to be the most suitable way.